This invention relates to a process for the formation of images using photopolymerisable compositions and to substrates bearing an image prepared by this process.
The preparation of images by photopolymerisation is well known. A photopolymerisable composition is applied as a coating to a substrate, which is often a resinous laminate bearing a metal on its surface. The coating is exposed to actinic radiation through a negative or other mask, and unexposed (and hence unpolymerised) portions of the coating are removed by means of a suitable solvent. The substrate may then be etched, removing only those parts of the metal surface which do not bear the photopolymerised coating. Such image-forming processes are useful, for example, in the manufacture of printing plates and printed circuits. Photopolymerisable compositions frequently contain an epoxide resin as a polymerisable component.
Benzenoid polyamines, i.e., compounds containing more than two hydrogen atoms attached to nitrogen atoms which in turn are directly attached to carbon atoms in benzenoid rings, are often employed for the curing of epoxide resins. It is customary to employ acidic accelerators such as salicylic acid or 2-methoxyethyl hydrogen maleate to reduce the time required to heat the epoxide resin with the benzenoid polyamine for curing; however, even at room temperature some reaction between the epoxide resin and the polyamine occurs when an acid is present and so acceleration is achieved at the cost of a shorter storage life for the curable composition.
It has now been found that an acid may be generated in a mixture of an epoxide resin and a benzenoid polyamine by incorporating certain substances and, when desired, exposing the mixture to actinic radiation. Since these substances, in the absence of such radiation, do not behave as acids, the mixtures have adequate storage lives. The irradiated mixtures may then be heated to bring about curing rapidly. It has been further found that the accelerating effect may be confined to parts of the mixture which have been struck by radiation; the parts which have not been so struck cure much less rapidly and so differential curing can be achieved by employing a heating step of suitable duration. Surprisingly, the accelerating effect in a layer of the mixture can be very closely restricted to areas which have been irradiated, that is to say, the liberated acid does not substantially diffuse into adjacent, unirradiated areas, and so by exposure to actinic radiation as through a negative it is possible to form sharp images by subsequently dissolving or washing away the uncured, unexposed areas of the mixture with a suitable solvent, the cured, exposed areas withstanding the effect of the solvent.
In our British Patent Specification No. 1,508,951 we have described a method for the preparation of prepregs, i.e., fibre-reinforced, heat-curable resinous composites, which method comprises
(i) impregnating a fibrous reinforcing material with a liquid composition containing an expoxide resin and a photopolymerisation catalyst therefor, and with a thermally-activated crosslinking agent for epoxide resin, and
(ii) exposing the impregnated material to actinic radiation such that the composition solidifies due to photopolymerisation of the epoxide resin through epoxide groups thereof but which resin remains substantially in the thermosettable state.
When desired, the prepreg is heated so as to bring about the curing of the resin.
Photopolymerisation catalysts which may be employed in that process for making prepregs include aromatic onium salts which liberate an acid on exposure to actinic radiation, especially diazonium, arsonium, phosphonium, sulphonium, selenonium, and iodononium complex salts, particularly the tetrafluoroborates and hexafluorophosphates. The thermally-activated crosslinking agent may be a benzenoid polyamine such as bis(4-aminophenyl)methane.
In our related British Pat. No. 1,587,536 we have described a similar process for making prepregs in which such a composition is exposed to actinic radiation before it is brought into contact with the fibres, viz.
(i) exposing to actinic radiation a layer of a liquid composition containing an epoxide resin as the sole photopolymerisable component and a heat-activated curing agent for epoxide resins until the said composition solidifies to form an essentially solid continuous film due to photopolymerisation of the epoxide resin through epoxide groups thereof but which epoxide resin remains substantially in the thermosettable state, and
(ii) bringing together the film so formed and fibrous reinforcing material under conditions such that the said film flows about the fibres and the components of the said film and the fibres form a coherent structure.
The prepreg is then heated to cure the resin.
In a third British Patent Specification No. 1,587,159, we have described a process in which similar compositions are used: there is described a method for bonding two surfaces together which comprises
(i) exposing to actinic radiation a layer of a liquid composition containing an epoxide resin as the sole photopolymerisable component and a heat-activated curing agent for epoxide resins until the said composition solidifies to form an essentially solid continuous film due to photopolymerisation of the epoxide resin through epoxide groups thereof but which epoxide resin remains substantially in the thermosettable state, and
(ii) sandwiching between, and in contact with, the two surfaces the film so formed and heating the assembly to cure the resin.
That an acid could be liberated by photolytic means to accelerate in situ the curing of an epoxide resin by an aromatic polyamine, and so provide the basis for a process for forming images, is unexpected. In British Patent Specification No. 1,330,100 there is described a process for preparing printed circuits including the steps of coating a support with a composition comprising an epoxide resin of epoxide requivalent weight 800 to 2000, an amine curing agent (which may be a benzenoid polyamine), a halogenated polycarboxylic acid anhydride, and a halogenated hydrocarbon solvent, exposing the coating to ultraviolet radiation in a predetermined pattern, heating the coating to cause hardening of the unexposed areas, and developing the coating to remove the exposed areas of the coating.
It will be immediately apparent that the process described in the aforementioned Specification No. 1 330 100 is for making positive images, whereas this invention provides a process for making negative resists. It is explained in Specification No. 1 330 100 that the chlorendic anhydride, i.e., 1,2,3,4,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid anhydride-the only halogenated polycarboxylic acid anhydride the use of which is illustrated--is believed to interfere with the thermal cure in the areas exposed to ultraviolet radiation. That patentee conjectured that, upon photolysis, chlorendic anhydride may release small amounts of hydrogen chloride which may form a salt with the amine curing agent, this salt being less reactive towards thermal cure, thus resulting in a differential solubility between the exposed and unexposed areas. The alternative explanation which was put forward is that reaction of the epoxide groups in the epoxide resin with hydrogen chloride or chlorendic anhydride occurs in the photolysed areas, slowing the rate of thermal cure.
Thus, the acceleration of cure taking place in irradiated areas, an essential feature of the present invention, when certain compounds are used, including those which are believed to form hydrogen chloride or other acids, is contrary to what would have been predicted from the disclosures of the above-mentioned British Patent Specification No. 1,330,100.